1. Field of the Invention
The present invention relates to tin oxide-based thick film resistor compositions which can be fired in an inert atmosphere, such as, nitrogen atmosphere, and which provide thick film resistors especially superior in reproducibility of resistivity and temperature characteristics.
2. Description of the Prior Art
Thick film resistors have been produced from a paste composition in the form of a paint or a paste consisting of conductive powder of metal, metal oxide and the like, and a glass frit dispersed in an organic vehicle. The composition is printed in a desired pattern onto an insulating substrate and fired. If necessary, trimming is conducted in order to obtain a desired resistivity. In the production of such thick film resistors, heretofore, ruthenium oxide-based resistors have been mainly employed, but, in recent years, tin oxide-based resistor compositions have been practically employed as thick film resistor materials which are firable in an inert atmosphere and provide resistors compatible with thick film conductors of base metals.
For instance, U.S. Pat. No. 4 322 477 (Japanese Patent Publication No. 59-15 161) discloses vitreous enamel resistors of tin oxide and glass frit. In order to regulate the resistance values, a certain heat treatment is conducted on tin oxide in a nitrogen atmosphere or a forming gas so as to cause controlled reduction of the tin oxide, prior to mixing it with the glass frit. U.S. Pat. No. 4 065 743 (Japanese Patent Publication No. 59-31 201) discloses resistor materials comprising a mixture of tin oxide and tantalum oxide or the products resulting from heat treatment of tin oxide and tantalum oxide admixed with a glass frit and states that the resistor materials have high resistivities with low temperature coefficient of resistance (hereinafter referred to as "TCR").
Further, there have been known resistor compositions comprising a mixture of two kinds of conductive powders, i.e., tin oxide and products resulting from heat treatment of and tantalum oxide, admixed with a glass frit. In these resistor compositions, since the resistivity can be adjusted by varying the mixing ratio of the two kinds of conductive powders without greatly varying the mixing ratio of the total amount of the conductive component to the amount of the glass frit, the TCR can be maintained at low levels over a wide range of resistivities and resistors having superior environmental characteristics, such as moisture resistance, high temperature characteristics and the like, can be produced.
However, since conventional tin oxide type resistors are disadvantageous in that since variation of resistance (i.e., dispersion of resistance) is large and TCR is instable, serious problems have arisen with regard to stability and reproducibility of resistor properties. Especially, since the resistors consisting of tin oxide and a glass frit exhibit highly negative TCR values, low TCR values close to 0 ppm/.degree. C. have been desired for the resistors.
Further, the tin oxide-glass system resistor materials also have difficulties in providing low resistance values, for example, in the vicinity of 1 kilo-ohm/square and, generally, low resistance values less than 10 kilo-ohms/square can not be obtained. In even the tin oxide-glass resistors including tantalum oxide, the resistance is controllable only in a high resistance range. When other oxide additives are added for resistance control, the addition is limited to small amounts and the properties of the resistors may widely vary. Therefore, variation of the resistance and TCR will be unfavorably large.